For fluid permeable covering sheets for absorbent articles of this kind, which in use are intended to be in contact with the body of a user, high demands are put on both softness and dryness.
However, it has proven difficult to accomplish a fluid permeable covering sheet having a soft, textile-like surface which remains dry even after repeated wetting when the covering sheet is being used on an absorbent article.
It is known to use nonwoven materials in order to achieve a soft and smooth surface on an absorbent article. In order to obtain a dry surface closest to the skin of a user, it is also known to use hydrophobic nonwoven materials which have been perforated so as to create apertures in the material which are larger than the interstices between the fibres in the nonwoven material.
Thereby, the fluid is passed through the perforated apertures, through the covering material and down to an underlying absorbent material layer.
In order to bond the covering sheet to an inside layer, it is common to use adhesive. However, the adhesives which are available at the present time are usually relatively hydrophobic, implying that fluid transfer from the covering sheet to the inside layer is hindered. When using a more hydrophilic adhesive, there is a risk that the adhesive will migrate into the hydrophobic nonwoven layer closest to the user which will cause the surface closest to the user to be perceived as wet after a first fluid insult. Another problem in connection with adhesively bonded material laminates wherein one of the layers are perforated is that the adhesive in the perforations may irritate the skin of the user during use. It is also a problem in the manufacture of the laminate that there is a risk that adhesive which happens to be applied in the perforations may be transferred to the transport band of the machine. A further problem in connection with absorbent articles exhibiting a perforated covering sheet which is adhered to an inner layer, is the risk of adhesive penetration when the articles are folded after manufacturing.
It is further known, for instance from EP 0,685,214 and EP 0,617,602 to bind together the covering sheet with an inside layer by melting the layers together in a bond pattern. In EP 0,617,602, a covering sheet consisting of a film is thermally bonded to an inside material layer by melting the layers together within specific bond areas. The bonds may, for instance, be circular bond sites.
However, it has been discovered that the covering sheet and the inside layer loose volume and thereby softness and skin friendliness by thermal bonding.
Furthermore, the thermal bonds make the surface of the covering sheet more stiff and thereby less comfortable to carry in direct contact with skin. Since the bonds reduce the volume, i.e. the thickness, of the covering sheet and the inside layer, the distance between the absorption body of the article and the body of the user is also reduced. This means that the risk of fluid penetrating back out of the article and wetting the body of the user is increased. It has also been discovered that the thermal bonds affect the fluid uptake in the covering sheet closest to the user so that the surface closest to the user's body is perceived as being wet after a first fluid insult.
Another problem when using absorbent articles of the above-mentioned kind is that they are exposed to different kinds of mechanical stress. Accordingly, in order to minimise the risk that the constituting layers are separated from each other during use, it is important that the layers in the absorbent article are sufficiently strongly attached to each other.
Furthermore, the bonding should be such that the fluid transfer ability between the constituting layers is good, that the article exhibits low rewet and high softness and conformability.